Learning CSS

A friend of mine is learning CSS. Like me, his standard MO when
learning something new is to just look at the source and start
playing around until you get it right. Mostly that works for CSS,
but I found that there were some subtle points that I didn't just
pick up and having a book helped. Here were two I found very useful:

CSS
Cookbook by Dan Cederholm and Christopher Schmitt is
your work-a-day O'Reilly book with lots of examples, good reference
material, and clear exposition. This one is handy for learning some
of the subtle tricks and seeing best practice.

What about reference material? For that I prefer asking Mr. Google.
If you need syntax, you can always find that online. I don't even
have a favorite site--I just type css textwidth (or
whatever) into Google and read the first site that pops up.